Modern printing techniques can be broadly categorized into two groups: analog and digital. Common analog printing techniques are offset lithography, flexographic, gravure and screen printing. Digital printing includes a number of techniques among which inkjet and electrophotographic printing are the most prevalent. Digital printing has an advantage over its analog counterpart in that printed output can be digitally altered, meaning that every printed page can be different. Moreover, digital printing methods are often cost effective, particularly at low run lengths (number of pages), and can have print quality comparable to analog printing methods, in many instances. In particular, since the mid-1980s, electrophotographic (EP) printing, commonly known as laser printing, has been a popular choice among consumers who demand high quality, professional looking printed communications. State-of-the-art commercial EP printers currently feature image quality that rivals lithographic offset printers and offer printing speeds that are compatible for virtually any print job.
Liquid electrophotographic (LEP) printing is a variant of EP printing that has superior image quality and the advantage of being compatible with a broad range of substrate types (coated and uncoated paper, plastic sheet, cardboard, folded cartons, shrink wrap and labels, for example). The ink used in LEP printing, as known as liquid toner, uses a dielectric carrier fluid and pigmented resin as colorant particles. Electrophoretic attraction of charged ink particles to a laser exposed photoconductor forms the image, which is transferred to an intermediate transfer medium prior to final transfer to the substrate. High quality output can be achieved at print speeds consistent with many commercial printing requirements.
The print quality from LEP printers is dependent on a number of properly functioning parts of the printer. For example, a plurality of development units carries the liquid toner of multiple colors to form the image. Liquid toner flows along a recirculation flow path in the development unit to provide an amount of liquid toner to a development roller that is subsequently transferred to the photoconductor for imaging. Excess liquid toner not provided to the development roller either continues circulating in the recirculation flow path of the development unit or is returned to an external reservoir where it is filtered and reconditioned (e.g., mixed with fresh ink) and then reused. Typical development units are sensitive to fluctuations in the amount and a makeup or replenishment rate of liquid toner at any given time and in any given location in the recirculation flow path. The sensitivity to the fluctuations may lead to formation of dark streaks in the printed image. Further in some instances, the sensitivity to the fluctuations may lead to build up of sludge within the development unit. The sludge build-up may result in more frequent replacement of the development units, which can be costly.